1. Field of the Invention
This invention relates to an automatic focusing (hereinafter referred to as AF) system suitable for a video camera system having interchangeable lenses.
2. Description of the Related Art
Recently, varieties of video equipment including video cameras are diffused and increased and video camera systems are accordingly designed to operate with interchangeable lenses.
To design a camera system using interchangeable lenses, a combination of a lens unit and a camera unit is selected according to the specific AF system for the camera system. For example, it is possible to adopt an ordinary combination of a camera unit which has main AF control functions, i.e., functions of determining a focused or non-focused state, detecting non-focusing values, i.e., the extent of defocusing and the direction of defocusing (near focus or far focus), issuing an instruction to drive a focusing lens and performing other operations, and a lens unit which has a driving circuit and a driving device for driving the lens and the aperture in accordance with instructions from the camera unit. In such a system based on a combination of a camera unit having control functions and a lens unit having driving functions, the lens can be driven by instructions issued from the camera unit alone, and there is a one-to-one relationship between the instruction and the driving. No problem is therefore encountered even if the system is designed to use interchangeable lenses. That is, this type of combination is adopted for a camera system using interchangeable lenses and designed in consideration of AF functions.
However, possible combinations of camera units and lens units are not always limited to the above-mentioned type. It is necessary to consider other types of combination. In some cases, it is possible that AF control operations will be arbitrarily started and the operation of the control system will thereby fall into confusion, and there is therefore a risk of an increase in the extent of defocusing. For example, in the case of a combination wherein the lens unit has a function of performing AF by driving the focusing lens on the basis of its independent determination procedure, it is possible that the lens unit will move the lens without waiting for the lens drive instruction supplied from the camera unit, resulting in a defocused state of the image.
It is possible to prevent this problem by adopting a method of inhibiting the main camera unit from performing lens control by outputting a signal to stop the AF control operation of the main camera unit. For an interchangeable lens system, however, it is necessary to designate in respective cases items of control information which are to be selected from control information supplied from the camera unit or items of control information which are to be ignored. This method therefore entails a number of problems including an increase in the number of items of control information represented by the control stop signal and a reduction in the lens combination capacity.
For development of a camera system using interchangeable lenses, it is necessary to solve various problems including those described above.